I haven't gotten it in the mail yet, it came out last week and it's on the way. The size 6 is back in stock now. Their rings tend to run pretty true to size, the reason the knuckle ring runs big I believe is because of the bend in the shape, where the inside is kind of hallowed out.. imagine putting the ring on a sizer, and it may read 7, but because there is that bend it gives a lot more room when it is actually worn. I have the knuckle ring too and had to bend it to like half its size to get it to fit.
If you normally wear an 8 I'd go with the 7.. since it's a 2 finger ring you don't want it to be too tight. I am a 5 and 4 in the middle and ring fingers where the feather ring is supposed to be worn and am crossing my fingers that I won't have to resize the size 6 I got.. 2 finger rings tend to be ok even when a little loose.